Physical inactivity increase risk for obesity, breast cancer, cardiovascular disease, osteoporosis, and type 2 diabetes. Obesity has been directly related to weight concerns and negative body image in adolescent girls, putting them at risk for eating disorders, lowered self-esteem, and depressive symptoms. Further, physical inactivity has been directly related to smoking uptake in adolescents. Compelling evidence now suggests that marked decline in physical activity in girls occurs during early adolescence. However, the documented decline in physical activity is not well understood, and existing theories have consistently failed to describe or predict adolescent exercise patterns. In previous work the Principle Investigator has developed the Theory of Meanings of Behavior, and shown that this is an important psychological element that explains behavioral changes in dietary changes in dietary patterns and sleeping habits in adolescence. In the current proposal, we hypothesize that this new theory will help to explain the decline in physical activity in adolescent girls and provide an innovative approach for interventions that will increase physical activity. This study proposes 1) an in-depth study of the determinants of physical activity in adolescent girls as well as an investigation of the mechanisms that link physical activity to various outcomes, focusing on how long girls interpret the meanings of physical activity, and 2) the development of culturally sensitive intervention to improve physical activity in Hispanic and Caucasian adolescent girls. Dr. Spruijt-Metz's short-term training goals are to acquire skills in body composition research, physical activity and nutrition in children and adolescents, and in advanced statistics. Her long term career goal is to become an independent researcher with an integrated program in adolescent health. The University of Southern California provides an outstanding environment for physical activity and health promotion research in children and adolescents. The department of Preventive Medicine has long fostered the careers of young scientists. The ability of the P.I. to interact with the sponsor, Dr. Goran, in addition to Dr. Anderson-Johnson, Dr. Stacy, Dr. Chou, and other excellent scientist at USC will promote her continued development as an independent researcher.